It took an ad campaign to sell Americans on its value.
1963 was a momentous year in America: President John F. Kennedy was assassinated, Martin Luther King Jr. delivered his “I Have a Dream” speech at the March on Washington and, somewhat less heralded by all but the most fervent postal historians, the ZIP code was introduced.
Since its founding in 1775, the post office relied on hand sorting based on local addresses to get mail where it was supposed to go. A piece of mail often went through 10 postal workers before making it to its recipient. But by the 1940s, the then-named Post Office Department realized its sorting system was not keeping pace with the growing population of the country it served.
For the rest of the story: http://nation.time.com/2013/07/01/the-zip-code-turns-50/